1. Field of the Invention
The present invention relates to a nanotube product such as a nanotube probe needle for AFM, a light emissive and absorptive nanotube probe, a nanotube heat generation probe, etc., in which a nanotube is protruded from a holder thereof, and more particularly to a sharpening method of nanotubes in nanotube products that produces nanotubes having sharpened tip ends which are used for operating a sample surface, thus improving the operation precision of a nanotube.
2. Prior Art
Recently in order to observe the surface structure of a material, an atomic force microscope (abbreviated as AFM) has been used. AFM has a semiconductor cantilever which is constructed by forming a protruding portion at the cantilever portion, so that unevenness (projections and indentations) on the surface of the material can be imaged by means of scanning the surface of the material, using the tip end of the protruding portion as a probe needle.
In order to make the semiconductor cantilever higher efficient, the inventors of the present application have invented a nanotube cantilever wherein a nanotube such as a carbon nanotube, etc. is fastened at the protruding portion of the semiconductor cantilever with a coating film or by means of an electric current fusion welding. The high quality AFM which can image the surface of the material in the precision same as the section diameter of the nanotube is realized, owing to use the tip end of the nanotube protruded downward as a probe needle point. This has been published in Japanese Patent Application Laid-Open (Kokai) No. 2000-227435 and No. 2000-249712.
Moreover, the inventors accomplished a heat generation probe that records information by means of an on-off mode which is formed with fusion welded holes. In this heat generation probe, a nanotube is fastened at the protruding portion of a semiconductor cantilever with a coating film or by an electric current fusion welding, a heat generation material is held at the tip end portion of the nanotube, and the on-off mode is formed with the fusion welded holes by means of heating the surface of an organic material in a pin-point fashion. This has been published in Japanese Patent Application Laid-Open (Kokai) No. 2002-243880.
The inventors further invented a light receiving and emitting probe that irradiates light on a sample surface in a pin-point fashion and receives light emitted from a sample in a pin-point fashion. In this light receiving and emitting probe, a nanotube is fastened by a similar means as described above at a protruding portion of a semiconductor cantilever, and a light receiving and emitting material is held at the tip end portion of the nanotube. This has been proposed in Japanese Patent Application No. 2001-81672.
In this way, various nanotube products have been developed such as a nanotube cantilever, a nanotube heat generation probe, a light receiving and emitting nanotube probe, etc.; and it is expected that various nanotube products that use such nanotubes will increase from now on.
These nanotube products are to positively utilize the characteristics that the section diameter of the nanotube is extremely fine. The section diameter of the nanotube is distributed in from several nm to several tens nm, and the extreme fineness of the nanotube diameter can be understood from the fact that theoretical minimum diameter of the nanotube is inferred to be about 1 nm. It is understood that the tip end of the nanotube product is excellent in the degree of fineness (sharpness), by comparing with the protruding tip end portion of the usual semiconductor cantilever, of which diameter is from several 10 nm to about 100 nm.
However, an arc discharge method or a chemical vapor deposition method (CVD method) produces a large quantity of carbon fine powder, and the carbon fine powder contains a large quantity of carbon materials other than carbon nanotubes. Therefore, the operation to pick out selectively nanotubes from this carbon fine powder is necessary, when a nanotube product is manufactured. More specifically, the additional operation which picks out an extremely fine nanotube from the nanotubes is necessary, when an extremely fine nanotube is requested in order to improve the degree of sharpness of a tip end.
It is difficult to see an extremely fine nanotube by an electron microscope, and even if the nanotube can been seen, it is difficult to take out one nanotube from a lump of nanotubes, as many extremely fine nanotubes often form a bundle. Furthermore, in practice an extremely fine nanotube cannot be used as a nanotube probe in many cases, since an extremely fine nanotube is too highly flexible, so that the nanotube is like thread trash.
For such a reason, a nanotube product usually uses a comparatively thick nanotube, which has a section diameter of ten and several nm or more so that the nanotube has somewhat rigidity. However, when the nanotube of such a large diameter is used, there is a limit in precision of size in the operation of the nanotube product.
FIG. 8 is an AFM measurement diagram imaging the surface of a sample by the nanotube cantilever which uses a usual large diameter nanotube. A nanotube cantilever 6 comprises a protruding portion 10 which is formed at a cantilever 8 made of semiconductor and a nanotube 12 which is fastened with a coating film 14 at the protruding portion 10.
The nanotube 12 is a large diameter nanotube that the section diameter is supposed to exceed 10 and several nm. In order to image the sample surface 22 of a sample 20, the tip end portion 12g of the nanotube is caused to approach to or contact with the sample surface 22 so as to detect the force acting on the nanotube 12 from projections and indentations on the sample surface 22 by a laser beam so on, and the information is imaged in a display.
In the sample surface 22, there exist many projections and indentations which contain widely from indentations with gentle inclination to indentations with steep inclination. On the other hand, the image of the indentation portion becomes unclear, when the diameter of the indentation portion is 10 and several nm or less, since the section diameter of the nanotube 12 exceeds 10 and several nm. In other words, when the section diameter of the nanotube 12 is large, as shown in the Figure, the tip end portion 12g cannot follow the inside of the indentation 22a, so that there exists a limit in the image precision of the sample surface. Namely, there exists the limit in precision as for the usual nanotube cantilever, according to a nanotube diameter. Such a fault exists in common in other nanotube products as well.
Next, the cause that the diameter of a nanotube becomes large is explained. There are a single layer nanotube (SWNT) and a multiple layer nanotube (MWNT). The single layer nanotube is such that a graphite layer surrounds a hollow portion in a cylinder shape and the multiple layer nanotube is such that multiple graphite layers surround a hollow portion in a concentric cylinder shape.
The above-described nanotube that theoretically minimum diameter is 1 nm is the single layer nanotube, and a graphite sheet (graphite net) is rolled strongly in a cylinder form with a minimum diameter. But, in practice, nanotubes manufactured by means of an arc discharge method or a chemical vapor deposition are almost multiple layer nanotubes, so that a procedure is necessary to find out extremely fine nanotubes from a large quantity of carbon fine powder. Besides, an extremely fine nanotube such as a single layer nanotube is like thread trash and is hardly useful.
As a result, a multiple layer nanotube with somewhat rigidity is used. Among the multiple layer nanotubes, if a tip end portion is closed in an acute angle fashion, the degree of sharpness of the tip end may be improved due to the acute angle shape of the tip end. However, the procedure to find out the nanotube which has the acute angle tip end is necessary, and even if obtaining acute angle nanotubes, the nanotube product of the same quality cannot be provided, if the acute angle of the nanotube is different in every nanotube. Of course, a high-resolution power cannot be gotten by a non-acute angle nanotube.
In other words, practically utilized nanotubes are multiple layer nanotubes which are selectively picked out from a large quantity of the manufactured carbon fine powder, and the number of graphite layers is usually from several layers to several ten layers or more. The section diameter of a nanotube which possesses many layers is necessarily large, and distributes in a wide range of 10 and several nm to several hundreds nm.
Namely, an extremely fine nanotube such as a single layer nanotube which is selectively picked out by worker""s perseverance and the zeal cannot be utilized due to the property like thread trash. On the other hand, there is a limit in the degree of sharpness of a tip end in the case of a multiple layer nanotube, of which section diameter is ten and several nm or more. Hence, even the selected nanotube with an acute angle tip end lacks in the stability of quality of a nanotube product as the acute angles are not uniform.
Accordingly, it is an object of the present invention to provide a sharpening method of nanotubes by which the tip end portion of a multiple layer nanotube with a large section diameter is sharpened.
It is another object of the present invention to provide a sharpening method that remarkably improves the operation precision of a nanotube product more than the ordinary method by means of sharpening the tip end portion of a nanotube at a later time, even if a nanotube with a comparatively large section diameter is used in a nanotube product.
The above object is accomplished by unique steps of the present invention for a sharpening method of nanotubes, and such steps comprises:
connecting the base end portion of a nanotube to an electrode with the tip end portion of the nanotube protruded from the electrode;
connecting the tip end portion of the nanotube to another electrode;
applying a voltage between the electrodes so as to cause an electric current to flow in the middle portion of the nanotube which is located between the two electrodes;
evaporating constituent atoms of the nanotube layer by layer from a evaporation starting region, which is located in the middle region of the nanotube, by the heat generated by the electric current, thus reducing the diameter of the evaporation starting region; and
cutting the evaporation starting region having the reduced diameter, thus forming a sharpened end on the nanotube.
According to this method, a nanotube with a sharpened tip end is obtained before a nanotube product is completed; and then the nanotube for which the sharpening treatment is made can be installed in a nanotube product. Accordingly, a nanotube product improved remarkably in operation precision is obtained. Moreover, according to the method of the present invention, the diameter of the sharpened nanotube tip end becomes the same as the diameter of most inside layer of the nanotube. Accordingly, a nanotube that possesses the rigidity same as a multiple layer nanotube as well as the tip precision (the sharpness of a tip end) same as a single layer nanotube is obtained. Moreover, by sharpening a recently discovered multiple layer nanotube that has a diameter of about 0.4 nm in its most inside layer with the use of the method of the present invention, a nanotube that has an improved tip precision up to 0.4 nm can be produced.
The above object is accomplished by another unique steps of the present invention for a sharpening method of nanotubes used in nanotube products in which the base end portion of a nanotube is connected to a holder with the tip end portion of the nanotube protruded from the holder; and the unique steps comprises:
connecting the tip end portion to an electrode;
applying a voltage between both end portions of the nanotube so as to cause an electric current to flow in a middle portion of the nanotube which is located between the holder and the electrode;
evaporating constituent atoms of the nanotube layer by layer from a evaporation starting region, which is located in the middle region, by heat generated by the electric current, thus reducing a diameter of the evaporation starting region; and
cutting the evaporation starting region having the reduced diameter, thus forming a sharpened end on the nanotube.
The above method advantageous in that a nanotube product formed with an un-processed nanotube is prepared beforehand, and the nanotube of the nanotube product is sharpened later. Accordingly, an improved operation precision of the nanotube is obtained, and the nanotube product is assembled while sharpening the un-processed nanotube. The nanotube product is satisfactory both in the rigidity of the nanotube and in the high resolution power and can be used in a more stable fashion than a single layer nanotube even in the case that the sharpened nanotube is used in a chemical force microscope after applying chemical function group to the tip end.
In the above methods of the present invention: the temperature of the central portion of the nanotube can be set higher than temperatures at both ends of the nanotube by way of using both end portions of the nanotube as heat absorption openings; and the central portion is set in the middle portion of the nanotube, thus allowing the central portion to be used as the evaporation starting region of the nanotube.
Therefore, the central portion of the nanotube can be sharpened without considering the evaporation starting region at all.
Furthermore, in the present invention, a defect, which is made on the surface or the inside of the nanotube during production of the nanotube, is detected by, for instance, an electric microscope, and the nanotube is arranged so that the defect is located in the middle portion of the nanotube. The defect portion is high in electric resistance. Thus, using the property that the defect portion has high electric resistance, the nanotube is formed with a sharpened end using the defect as the evaporation starting region.
In the present invention, the evaporation starting region can be artificially formed at a desired position in the middle portion of a nanotube. The artificially formed evaporation starting region is high in electric resistance. Accordingly, using the property that the evaporation starting region made artificially has high electric resistance, constituent atoms in the nanotube are evaporated from this artificially formed evaporation starting region, thus obtaining a sharpened end on the nanotube.
Furthermore, in the present invention, a defect can be artificially formed at a desired position in a middle portion of the nanotube by means of a mechanical operation or a particle beam irradiation. The artificially formed defect is high in electric resistance. Accordingly, using the property that the defect portion has high electric resistance, constituent atoms in the nanotube are evaporated from this artificially formed defect portion, thus sharpening the tip end of the nanotube. It is an advantage of this method that particle beam irradiation such as an electron beam or an ion beam, etc. is easily performed in order to form the defect by an existent electron microscope or an existent ion beam apparatus.
In the present invention, the middle portion of the nanotube can be curved by an external force. The maximally curved portion of the middle portion is high in electric resistance; and using the property that the maximally curved portion has high electric resistance, constituent atoms in the nanotube are evaporated from this maximally curved portion, thus forming the nanotube with a sharpened end thereon.